Yes, and usually more than once
Most babies' hair changes in two stages. The first comes early. Most hair at birth is dark, and it is temporary: it begins to shed by about 1 month, and the permanent hair starts to appear by about 6 months. That new hair can be an entirely different colour.
The second change is slower. Light hair commonly darkens through childhood and again around puberty, so the colour at 6 months says little about the colour at 16.
The first months: shedding and regrowth
Almost all babies lose some, and possibly all, of their hair during the first few months. Paediatric skin specialists at the American Academy of Pediatrics call this normal and expected, and the hair grows back.
Babies do it in different ways. Some lose their newborn hair slowly while the permanent hair is coming in. Others lose it quickly and are bald for a short time.
A bald spot on the back of the head is common too. It often comes from rubbing the head against the sleep surface, even when the rest of the hair stays, and new hair covers it within a few months.
Because almost every baby sheds hair, there is usually no need to see a doctor about it. If hair loss carries on or worries you, talk to your baby's doctor or health visitor.
Childhood and puberty: hair often gets darker
MedlinePlus Genetics notes that, particularly in people of European descent, light hair often darkens with age, and blond children often have darker hair by the time they are teenagers. Researchers think some of the proteins that make hair pigment switch on as children grow, perhaps in response to the hormonal changes around puberty.
A study of 476 children aged 6 to 13 shows how common this is. Of the children who had been blond at age 2 to 3, about 7 in 10 had brown hair by the time of the study. Even a DNA test built to predict hair colour was often fooled: for most of these children it still predicted blond.
The same study found that the DNA test picked out red hair far more reliably than blond or brown. Red depends mainly on one gene, MC1R, while blond and brown shift as children grow.
If you want to follow the changes month by month, the baby age calculator tells you exactly how old your baby is.
What decides hair colour
Hair colour comes from two pigments. Eumelanin gives black or brown hair, and pheomelanin gives red hair. The type and amount of each are set by many genes, most of which are still poorly understood. A large study of UK adults linked blond hair to more than 200 genetic variants and found a smooth range from black through dark and light brown to blond, rather than neat boxes.
What the parents' colours can tell you
Both biological parents pass on genes for hair colour, and some of them stay hidden. That is why the baby hair colour calculator above works with odds rather than certainties.
It uses a simple two-gene model: one gene for dark or light pigment, where dark wins, and one gene, MC1R, for red, which only shows when a child gets a red version from both parents. It describes the colour your child is likely to grow into, not the colour in baby photos.
For example, for two dark-haired parents with no red hair in either family, the model gives about 91% black or brown, 8% blond and 1% red. Real families vary, so treat these numbers as a fun estimate, not a genetic test.
A note on red hair
Red hair works a little differently. It depends mainly on MC1R. One copy of MC1R with a change that switches it off can give strawberry blond, auburn or red hair. When both copies are switched off, the hair is almost always very red.
Because the red version can pass quietly through parents who do not have red hair, a red-haired baby can surprise everyone. If one of you has red hair, see one dark-haired and one red-haired parent, or two red-haired parents. If red hair runs in either family, tick "Someone in their family has red hair" in the calculator: it raises the estimated chance.
Grey hair is a separate story
Greying has nothing to do with these early changes. Almost everyone's hair turns grey with age, when the hair follicles lose the ability to make melanin. When that happens is partly hereditary and varies a lot from person to person.